Motor vehicle electronics have changed drastically in the last thirty years. Prior to the early 1980s, computer electronics had not matured sufficiently to allow incorporation into vehicles. Electrically operated components were attached to the main electrical system via wire harnesses strung throughout the vehicle.
A typical late model vehicle 10, illustrated in FIG. 1a, has an electrical system 12, including wire harnesses 14 connecting, e.g., a battery 15, starter 16, lights 18, and console 20. Shown in diagram form in FIG. 1b, the electrical system 12 includes starter 16, an alternator 30 and battery 15 supplying power to the vehicle. The power is usually controlled through manual dashboard or door mounted switches and relays 34, with peak power transient protection managed through a fuse box 36. When a manual switch or relay 34 completes a circuit, the power flows to the associated motors 38, electronics 40, lights 18, or heaters 44.
Electronic technology has advanced to the point that most automobiles implement advanced electronics technology to control everything from ignition systems to antilock braking systems. As more electrical components are added to a vehicle, more wires must be added to the wire harnesses, increasing their thickness. Thicker wire harnesses lead to several design issues, most notably difficulty in installing wire harnesses throughout the vehicle. For example, there is a practical limit to how many wires can be strung through vehicle door canals before the vehicle door's operation is affected. Including individual wires for window controls, locks, outside mirror controls and other switches and lights can make proper opening and closing of the doors difficult. The future sees even more electronics being added to vehicles.
To resolve this problem, manufacturers have begun to use multiplexing communications technology. With multiplexing technology, numerous electronic modules are linked by a single signal wire in a bus typically also containing a power supply bus wire. Far fewer wires are required than those found in the conventional arrangement discussed above.
A typical multiplexed circuit 50, illustrated in FIG. 2a, includes a control processor 52 and a multiplexing unit 54. A wire harness 56 contains a communication bus also known as a control bus, a power supply bus, and a ground bus 58 which connect vehicle components 64 and 68 via cables 60 and connectors 61. The vehicle components 64 communicate digitally with the control processor 52 via multiplexing units 62 and 66. The multiplexers 54, 62, 66 send and receive digital signals consisting of commands and status data. A command is a signal that actuates a component, and status data is a signal that indicates the component's state or condition. In the typical system, digital commands are broadcast to all components over the bus wire 58. Each command has one or more addresses appended to it, so the components can tell which to ignore and which to read. Depending on a component's complexity, component status data is broadcast to the control processor 52 so real or near real-time operations, and fault detection and recovery can be performed. Any power the vehicle components need is drawn from the power bus.
The likely applications for multiplexed buses in vehicles span everything from one-shot tasks (actuating a door lock) to intricate activities (engine control). To cope with this broad range, the Society of Automotive Engineers (SAE) has defined; in specification J2057, available from the SAE, in Warrendale, Pa.; three vehicle network categories called Class A, Class B, and Class C. The system described in this specification is one example of how multiplexing may be used to simplify and improve control of electrical systems in a vehicle.
An example of a vehicle 70 employing a 3-bus control system is illustrated in FIG. 2b. The vehicle is provided with three control buses 72, 74, and 76 distributed throughout the vehicle. Bus 72 is a Class A bus, bus 74 is a Class B bus, and bus 76 is a Class C bus. The Class A bus 72 is connected to vehicle components via interfaces 78. The Class A bus operates fundamentally as described with reference to circuit 50 above. The hardware components necessary to interface with the Class A bus are the least expensive as compared to the hardware required to interface with Class B and Class C buses. The vehicle control processors need send only simple commands and receive simple telemetry from electrical components actuated using the Class A bus.
The Class B bus 74 is connected to vehicle components via interfaces 80. The Class B bus also operates fundamentally as described with reference to circuit 50 above. The Class B bus has a "mid-range" bandwidth that is wider than the Class A bus but not as wide as a Class C bus. The Class B bus meets all the Class A bus requirements and can therefore interface with simple components as well as more complex components such as alternators and electronic ignitions. The hardware required to interface with the Class B bus is more expensive than that required for the Class A bus but not as expensive as the Class C interface hardware.
The Class C bus 74 is connected to vehicle components via interfaces 82. The Class C bus also operates fundamentally as described with reference to circuit 50 above. The class C bus has a "wide" bandwidth that is wider than both Class A and B. The class C bus meets all the Class A and B bus requirements and can therefore interface with less demanding components as well as components requiring real-time control such as antilock braking systems and active suspensions. This allows a vehicle's control processor to coordinate components to improve efficiency as well as safety. Motorola Corporation offers multiplexers specifically designed to meet the Class A, B and C, SAE J2057 standard. Control processors are standard in the industry and are familiar to those skilled in the art.
Manufacturers such as Daimler-Chrysler have implemented simple multiplexed control systems in automobiles. Using a single bandwidth bus, such as the Class B or Class C bus, electrically controlled components are operated. A typical vehicle 90, illustrated in FIG. 2c, shows a single class bus 92 installed throughout the vehicle 90. The bus 92 is connected to vehicle components via interfaces 94.
This implementation necessarily requires the manufacturer to use the same interface hardware for all electrically controlled components within a vehicle. In ultra-luxury vehicles such as Mercedes-Benz, the manufacturer may use fiber optic cabling with a Class C type bus to distribute information to and from control processors allowing vehicle component control. This necessarily increases the overall vehicle cost because the interface hardware costs increase as the bus bandwidth increases. In the single bus implementation, the most expensive hardware must be used even for the components that otherwise could be controlled using a narrower bandwidth bus. Clearly, if vehicle components can be matched with bus bandwidths appropriate for their needs, fewer high cost interface multiplexers would be required, lowering the overall vehicle cost. However, as shown in FIG. 2b, this requires distributing three separated harnesses throughout the vehicle.
Another emerging issue with vehicle design is multi-voltage systems. Novel electrical equipment, like electromechanical valve actuators and active suspensions may eventually triple the aggregate electrical power demand in some cars from 800 W today to an average of 2500 W and a peak value above 12 kW. That power can be more effectively distributed and utilized at voltages much higher than the 12 V dc currently used. Therefore, a system that can control and monitor vehicle components, using an integrated bus with selectable bus bandwidths and identical interfaces is desirable. Additional capabilities such as component bus bandwidth matching and voltage conversion units to accommodate multi-voltage systems adds to the system desirability.